Link Access Procedure, Balanced (LAPB) is protocol that is usually required for data link layer. It is version of High-Level Data Link Control (HDLC) in Asynchronous Balanced Mode (ABM). LAPB use of P/F bit is somewhere different from all of the other protocols. LAPB basically uses three types of data frames. Each of these frames has different functions and purposes.
These three types of frames are given below :
1. I-Frames :
These frames are generally known as Information frames. It is basically used to carry information or data of upper layer. It includes two different sequence numbers i.e. N (S) that is used to send sequence number and N (R) that is used to receive sequence number which is the expected value of what N (S) will usually take in the next I-frame that is going to be received. This frame also includes X.25 PLP Packets and is always commands.
2. S-Fames :
These frames are generally known as Supervisory frames. It is used to provide control information. This frame only includes N (R). Using S-frame, there basically three different types of messages that are sent. S-frame usually controls data flow with RR, RNR, and REJ Frames.
- Receive Ready (RR) –
RR is used to acknowledge current frames that are being received and also indicates which of frame is expected to be next. This frame can be either command or response out of which response RR’s are required to acknowledge data frames that are being received and also in reply to command RR’s. On the other hand, Command RR’s are required when force response from peer and are also basically required when link is idle simply to check whether it is still active or not.
- Reject (REJ) –
REJ is used to reject signals of frame usually by receiver. This is basically required when I-frames are received with unexpected sequence number. Unexpected sequence numbers simply indicate to peer that I-frames are required to be transmitted again or retransmitted.
- Receive Not Ready (RNR) –
RNR is used for control of data flow and also required to indicate that receiver is not at all ready to receive any data that is extra or additional and transmitter has to wait anyhow until receiving ready message is sent or transmitted. These are usually transmitted or transferred when local end of link is temporarily not been able or ready to receive any further information frames.
2. U-Frames :
These frames are generally known as Unnumbered frames. It is used to provide unsequenced control information. This frame does not include any type of sequence numbers at all. It is also required for link setup, disconnection of link, and error reporting. It is basically required to establish or develop and maintain or handle communication with SABM, UA, DISC, DM, and FRMR.
- Disconnect (DISC) –
This command usually disconnects or terminates link or connection.
- Set Asynchronous Balanced Mode (SABM) –
It is used to develop DTE and DCE link in Normal i.e. basic mode (modulo 8) sequence numbers. It is needed to be acknowledged by UA Frame so that link gets to enter information or data transfer.
- Unnumbered Acknowledgment (UA) –
It is basically transmitted in response to SABM frame simply to set link to information transfer.
- Disconnected Mode (DM) –
This is basically response frame. It is being transmitted or transferred in response to command frames in disconnected phase when link is not ready to enter information transfer.
- Frame Reject (FRMR) –
It basically reports an error condition. It is also response frame that is being transmitted whenever any error occurs that might be resolved by resetting data link.
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